Alkylphenols are materials of commerce desirable for their antioxidant properties. Many members of this class have commercial utility in such applications as antioxidants and stabilizing agents for fuel oils and antioxidants for foods of diverse type. Among the phenols which are antioxidants the ortho-alkyl- and ortho, ortho-dialkylphenols appear to be superior. That is to say, the ortho-alkylphenols and ortho, ortho-dialkylphenols seem to be better antioxidants than their isomers. There is a corresponding need to prepare such ortho-alkylated phenols with relatively high selectivity and yield.
The usual method of preparing alkylphenols is to alkylate phenols with an olefin, alkyl halide, or alcohol in the presence of an alkylating catalyst which generally is a Lewis acid. Catalysts which have been employed include strong inorganic acids (sulfuric acid, phosphoric acid, and hydrofluoric acid to name a few), strong organic acids (for example, sulfonic acids and cationic exchange resins bearing such acid functionalities), metal halides (boron trifluoride, aluminum halides, and zinc halides are exemplary) and inorganic oxides such as alumina and silica. A deficiency in all such methods is their limited selectivity for alkylation at available positions ortho to the hydroxyl group vis-a-vis alkylation at other available positions. This limitation is particularly acute where the phenol bears another substituent, e.g., an alkoxy moiety, which is comparable to the hydroxyl group in effecting ortho-alkylation. Since 2-t-butyl-4-alkoxyphenols are generically especially desirable antioxidants, as exemplified by the broad usage of 2-t-butyl-4-methoxyphenol, popularly known as BHA, the lack of selectivity discussed above has great economic impact.
Some instances of the rearrangement of alkyl phenyl ethers to the isomeric alkylphenol have been reported. For example, U.S. Pat. No. 2,289,886 discloses that alkyl phenyl ethers when treated with hydrogen fluoride afford both the isomeric alkylphenol and the dealkylated phenol. More recently U.S. Pat. No. 4,283,572 describes the rearrangement of nonyl phenyl ether to a mixture of phenol, monononylphenol, and dinonylphenol. Such sparse reports are in marked contrast to the well-known thermal rearrangement of allyl phenyl ethers to allyl phenols (Claisen arrangement) where the allyl group migrates selectively to an ortho or, less often, to a para position.
We have made the remarkable discovery that alkyl phenyl ethers undergo a thermal rearrangement in the presence of an alumina as catalyst to afford the isomeric ortho-alkylphenols with high yield and good selectivity. Not only is the thermal rearrangement of an alkyl phenyl ether to an alkylphenol without precedent as a general phenomenon, at least at the temperatures used herein, but the regioselectivity of the rearrangement to afford an ortho-alkylphenol is completely surprising. That this regioselectivity persists when the alkyl group is a t-butyl moiety and when the aromatic ring bears another alkoxy group para to the t-butyl ether moiety is totally unexpected, and is the basis of our process of making 2-t-butyl-4-alkoxyphenols.
Such a method of making an ortho-alkylphenol has many advantages over the prior art methods in addition to the observed regioselectivity. One advantage is formation of the ortho-alkylphenol at a substantially lower temperature than was previously possible. That is to say, the rearrangement occurs at a temperature lower than that necessary for alkylation of the phenol with, for example, an olefin using an alumina as the alkylating catalyst. Since the alkyl phenyl ether may be prepared from a phenol under relatively mild conditions, our discovery makes possible a two-stage preparation of an alkylphenol via 1) formation of alkyl phenyl ether followed by 2) rearrangement of the ether, both reactions proceeding under substantially milder conditions than direct alkylation of the phenol.
Another important advantage of our method is the strict control it affords over the degree of alkylation. Thus, since only the alkyl group of an alkyl phenyl ether migrates to the ortho position our process is tantamount to exclusive monoalkylation. Because monoalkylation of phenols, e.g., 4-methylphenol, often is plagued by overalkylation to give undesired products, e.g., 2,6-di-t-butyl-4-methylphenol, our method is unique in affording exclusive monoalkylation.
Still another advantage of the method which is our invention is that it affords products which sometimes are not otherwise readily available. For example, ethers of an (2-alkylphenyl) alkyl ether undergo rearrangement to the isomeric 2,6-dialkylphenol with great specificity, whereas direct alkylation of the same 2-alkylphenol may fail to afford the desired 2,6-dialkylphenol, or do so only in relatively poor yield.